Just saying, this will be based off of current details in the KR server for Closers Online.

This guide will assume that you have Force Connection skill option cubes on all of your skills, and mainly use skills in the air.

This guide is also subject to changes, and may change depending on new changes to Seha.

As of 5/1/17, this guide is up-to-date.

General Overview
Seha is a melee-mage hybrid character, utilizing both melee attacks and medium to long ranged projectiles, shredding through armor and damage reduction with his signature [True Damage]. While his skill damage is quite average, his true damage passive makes up for it, allowing him to deal more damage to enemies with damage reduction. He probably has the most unique passive in the game, as he is the only one that can really utilize true damage ([Phase Power Release] is true damage on all characters by default).

Skill Guide

Trainee

Discharge
On the ground, Seha will slash forward, and then blast enemies into the air, and follow up with a homing blast. In the air, Seha will immediately detonate a small area in front of him, and then follow up with a homing blast.

This is his bread and butter skill. With a low cooldown, decent damage, and fast speed, it's a great way to clear mobs and deal damage to bosses. Unfortunately, the range on it is kind of limited, and the chase blast is strange to use sometimes (a recurring theme with chase blasts) because of how it pushes you into the enemy that you are chasing. Thankfully, the chase blast has a large hitbox, so in a laggy scenario, you will probably get both hits either way. It also has the added benefit of giving you 1 use of [Rush], Seha's primary dashing skill.

On the ground, it's very slow and kind of weak, as about a third of the full damage is turned into a normal sword slash that doesn't proc true damage. However, in the air, the initial startup of the skill is removed, and you get all of the damage as true damage. It turns from a slow skill on the ground to a fast damage dealing skill in the air. If possible, you should never use this skill on the ground.

In PvP, this skill is great for its low cooldown, and the 1 use of [Rush] in PvP is a godsend. Even through latency, it is actually quite reliable mid-combo, which is surprising considering your character follows your enemy with the chase blast. The only thing you'd have to watch out for is when enemies sometimes don't suffer through hitstun for long enough, and they hit the ground very quickly. Unfortunately, there isn't really much you can do about that, so you just have to watch out for it. Unlike in PvE, you can use [Discharge] on the ground in order to make use of its somewhat slow animation for super armor frames.

Burst
On the ground, Seha throws out a straight-line wave of fire, with one final blast at the end of the line. In the air, Seha will throw a grenade-esque ball of fire that detonates if it collides with a wall, the ground, or an enemy. The skills [Instant Awakening] and [Phase Power Release] will give you 1 use of this skill. ([EX Discharge] will also give 1 use of the skill, but only in PvE)

This skill replaced [Reversal] when Seha's Special Agent patch was released, and by god, it was probably the best change Seha has ever received in the history of the game. Whereas [Reversal] was only useful for extending combos or giving you super armor as you jumped into the air because of its HORRRRIBLE damage output, this skill replaces what little utility [Reversal] used to have with pure damage. This skill can have two uses, with a fixed cooldown of 8 seconds.

Learning how to use this skill is essential to Seha's DPS. At level 20, the enhanced blast will deal 3749%, and since you can reset the cooldown of this skill, it means that with a full rotation of Seha's skills, this skill alone can deal upwards of 15000%. Although the ball of fire has a somewhat narrow hitbox, the explosion has a decent hitbox, which means that it can hit multiple enemies. But unfortunately, it can get body blocked if you are trying to target multiple enemies, because it will explode as soon as it makes contact with an enemy. Although it may be uncomfortable to use at first, learning how to use this skill with [Rush] will be essential in order to deal as much damage as possible to bosses.

In PvP, [Burst] acts as a restand, as well as dealing very good poke damage. It only has one total use in PvP, but it can still be reset by [Instant Awakening] and [Phase Power Release]. As this skill has infinite vertical range (as it only explodes on contact with enemies or the ground), this skill can be used as a safe poke toward enemies from up high. Also, if they do get launched by this skill from being caught off guard, you can easily follow up with another restand or an attack, because it has large amounts of hitstun, and knocks down for a long time.

***EDIT***
Seha's recent balance patch (released with SC Harpy) changed Burst to be a line of fire in the air as well, instead of a grenade. What this means is that instead of hitting the first thing that it impacts, it hits everything within the line, increasing its overall AoE and usability in both PvP and PvE.

Rush
Seha dashes forward, dealing damage to enemies in his path. On the ground, holding upwards will cause Seha to slash upwards into the air, and in the air, holding downwards will make him spiral downwards and land with a small shockwave in front of him. During the normal dash, you can control your character's y-axis position by holding up or down (holding up or down too early will cause you to do the other functions). In the air, Seha will pass through enemies that he dashes into, but on the ground, he will be affected by enemy collision. You can have up to 4 5 uses of [Rush]. This skill can also gain 1 use when Seha uses [Discharge], [Instant Awakening], [Phase Power Release], or [EX Discharge].

This is Seha's main dash skill. It has so many different uses that I would actually consider it borderline OP, even if the damage is really bad. Its utility and ability to weave up and down while passing through enemies is extremely useful, and you also use it in order to go from a grounded state to the air immediately, while in super armor. Seha's playstyle revolves around him flying around the enemy, dealing damage from above. This skill allows him to dash out of potential danger, and it lets him get behind an enemy so that he can proc Back Damage as well.

This is also used to transition from a skill that brings you back to the ground back into the air ([Shockwave], [Point Blank Strike], [Flame Pulverization], [EX Shockwave]), so that no time is wasted, and so that you won't have the risk of being caught without super armor while jumping normally. It also acts to cancel the end lag of these skills, so that you can immediately be back in the air instead of having to wait for the skill to end naturally.

In PvP, the max amount of uses you can have at once is capped at 2. It's still a very good dash, and it can still be reset by [Discharge], which has a very low cooldown itself. Running out of dashes to escape bad situations or engage enemies isn't really a big problem for Seha.

Also, if used with the downwards spin variation in the air, [Rush] is a restand.

***EDIT***
In a recent patch, this skill's master cube was changed to give one extra stack, bringing up the total dash stacks from 4 stacks to 5.

Instant Awakening
Seha temporarily unlocks his hidden potential to give himself a buff with Physical Damage, Movement Speed, and Physical Critical Damage. Also, in PvE, there will be a small burst of damage (~900%) in a small AoE around Seha.

This skill has a massive cooldown (30 seconds), but the buff also lasts for 30 60 seconds, so it just means 100% buff time. Hooray!
Speaking of the buff itself, it's a very good buff, and at level 10, it gives 10% Damage, 20% Movement Speed, and 10% Physical Critical Damage. Unfortunately, it's really annoying because you can't cancel this skill until the flash appears in his hand, so it takes a VERY long time in order to receive the buff. On the bright side though, you will have damage reduction during the animation, and if you need a lot of super armor frames, it's good for that too.

In PvP, this skill should be used either to reset the cooldown of [Burst], tank damage during the damage reduction, or resist grabs (as this skill has the grab-resist property).

***EDIT***
Recent balance patches for Seha have changed this attack to be invulnerable in PvE, and the buff will last 60 seconds in both PvP and PvE.
Using this skill will also reset the cooldown of Discharge and EX Discharge.

Finishing Move Storm Blade
On the ground, Seha unleashes a flurry of slashes before launching enemies upwards and blasting them away, following up with a homing blast. In the air, Seha will detonate the area in front of him, and then follow up with a homing blast.

In a very recent patch, the range on this skill in the air was increased, which made this skill much easier to actually land. Unfortunately though, it has a mediocre damage output, so this skill is basically just a place holder while your other skills are on cooldown.

In PvP, this skill is very reliable, both in the air and on the ground. On the ground, it has enough time-stop for Seha to slash about three times before the enemy can move away, which means that you can eat up i-frames with this skill, as i-frames such as the get-up invincibility and the i-frames from [Force Cancel] will continue, even when the player cannot move, meaning that time-stop will make it so that the i-frames will end before the enemy can react. In the air, it is very good for damage (as in PvP, this skill does a lot of damage), and the range is quite good, meaning that if the enemy is without a [Force Cancel], you can dash in and use it to break any super armor they might have.

Intern

*Seha has a hidden passive when he job advances to Intern, which grants him 10% Aerial Damage.

Pulse Cannon
Seha shoots out a blast of fire in front of him, following up with another blast.

This skill is kind of awkward to use because of how far back it moves Seha when it's used, but it has very decent damage, and it's able to penetrate through mobs, so it's a good clearing skill as well. Other than that, it's quite simple, and there isn't too much more I can elaborate on.

In PvP, it has extremely long range, being able to go off screen, so it should be used to harass and deal damage to enemies from far away.

***EDIT***
This skill was given the Damage Reduction property in a recent update.

Shockwave
Seha drives his sword into the ground, launching enemies in the area around him, and follows up by pulling enemies in the area toward his position.

This skill is extremely fast, but it is unfortunately one of Seha's few skills that do not deal true damage (something that is rectified once you advance to Special Agent). It has a great AoE, and it pulls enemies to be either in front of you or behind you. Overall, this skill is great for gathering enemies and dealing some fast damage.

In PvP, the quick AoE from this skill is great for catching people off guard, and the fact that it pulls enemies towards you makes it a pretty easy combo if you get it. However, be wary of which side the enemy will be on after you gather them. You can rectify this issue if you use a skill that has another aoe around you, but you just have to be careful with this skill. Otherwise, it is a very good poke and catching tool.

Phase Focus Blade
Seha enhances his gunblade with phase power, enhancing his attack range and attacks, and follows up with two spin attacks.

This buff is amazing, giving a LOT of attack stats, as well as increasing Seha's attack range. With enough cooldown reduction, you can also have this buff 100% of the time, so this buff is an extremely big core part of Seha's gameplay. With the Master Cube, Seha will also charge part of his Phase Gauge when he uses this skill. Other than giving the extremely useful buff, the skill itself is quite simple, so I won't elaborate further.

In PvP, this skill can be used to tank some damage, as well as resist grabs (as in a recent patch, this skill was given grab-resist properties in PvP) while giving yourself the buff. Unfortunately, it doesn't give you the increased attack range, but the buff is still useful.

***EDIT***
In a recent patch, this skill was revamped to be invulnerable in PvE (still damage reduction and grab resist in PvP), the charge time was increased to ~4 seconds, and they increased the buff time to 60 seconds. However, the extension was removed. This means that SEHA FINALLY HAS AN I FRAME SKILL!!!!!!!!... in dungeons. But hey, at least he got one right?

Finishing Move Starlight Sword
Seha dives into the ground, and follows up with an uppercut.

This skill is amazing.

It has very good damage, dealing upwards of 12000% of true damage, and about 6000% of normal physical damage. It is also extremely fast when used in the air, with almost nonexistant start-up time. Because of the strange angle that Seha dives in, it is somewhat strange to learn to use, but once you learn how the collision works with it, it becomes one of Seha's best bossing attacks. The trick is to aim for the bottom half of the boss when you use it, and it will make you stop once you hit them.

In PvP, this skill can be used in two ways.

One way is to use the aerial version and try to snipe enemies, dealing a large amount of damage very quickly. Doing it this way makes it hard to react to, so some people may not use [Force Cancel] until most of the damage is done.

The other way is to use the time-stop to hit enemies while on the ground. The grounded version is much slower to start up, but it has a LOT of time-stop on it. So much so, that if you use it as soon as they start blinking from get-up, it will actually eat up their i-frames and hit them unless they instantly use another skill with i-frames on it. This method is much safer than the aerial version, but it sacrifices some damage for utility.

Focus Fire* (Locked Skill)
On the ground, Seha fires two blasts upwards after launching enemies and follows up with a homing blast. In the air, Seha will shoot three blasts diagonally downwards, and then follow up with a homing blast.

Before the main overview of this skill, I will first acknowledge that you must unlock this skill via a skill ticket, either through daily tokens or cash.

On the ground, this skill is very awkward with the launch, so I wouldn't really use it.
In the air though, this skill becomes a very strong bursting skill. It's fast and starts off with projectiles, so it can also be used as a gap closer if you are high up.
After the homing blast, you can also immediately input [Finishing Move Storm Blade] to ensure that you will hit the skill.
If you are close to the ground after the homing blast, you can also follow up with [Finishing Move Starlight Sword] or [Finishing Move Inferno Blade] as well.
If you chase an enemy that is grounded, it may look as though you will be grounded immediately when you homing blast, but if you input a skill immediately after homing in, it will come out as an aerial skill, so if you choose to do so, you will not have to go through the pain of jumping or using [Rush] to go back into the air.

Official Agent

Sky Slash
Seha uses his gunblade to slash, and rises into the air, emitting flames from his gunblade while spinning, and follows up with a homing blast.

While slow to start up, this skill does a large amount of damage, and it is very reliable to use. You can think of this skill as a [Discharge]-esque big-brother of some kind.

In PvP, this skill is great for trading because of its large amount of damage. Also, if you are above the enemy, you can use this skill to hit them with the very first slash, and if it catches them off guard, it will launch them into the rest of the skill, where you can convert the skill into a full combo.

EXTRA TECHNIQUE

(As this technique has no real name in English, I will dub it as the "Spin Cancel")
The spin cancel allows you to cancel [Sky Slash] during the second part of the skill, when he is spiraling upwards, to still get the rest of the spin to come out. Even though the animation is canceled, the damage and hitbox from the flame-spin portion of the attack will continue, which allows you to use [Sky Slash] and use other skills as soon as the spin starts, to get the rest of the spinning damage while you are doing other attacks.

Point Blank Strike
Seha lunges forward, grabbing enemies in his path. If he successfully grabs enemies*, he blasts them with his gunblade at point blank range, and follows up with another blast (Two blasts if the Master Cube is unlocked). If used in the air, he dives forward. If Seha fails to grab an enemy, he shoots a blast backward or forward, depending on the direction the player is holding (Two blasts if the Master Cube is unlocked).

*During a successful grab, Seha will have i-frames throughout the entire animation of [Point Blank Strike], along with the extension.

This skill is AMAZING. High damage, high utility, good range on the blasts, and i-frames on successful grabs. It has everything that someone could really want. Although the grabbing animation is kind of slow (but cool looking), being able to have i-frames for about two to three seconds can actually save your life, being able to just i-frame through some attacks.

Usually for bossing though, you want to intentionally fail the grab, because for some reason, the damage that you get for failing a grab is only very slightly lower than actually successfully grabbing, and it is much faster. Since you pass through enemies when you use this skill, you want to try and pass through enemies to hit them from behind in order to receive more bonuses from your situational damage.

In PvP, this skill is actually insane. Although it may not be the best grab due to its very narrow hitbox, it has amazing damage, with or without a successful grab. You can also use it as a pseudo-dashing skill because of how the skill starts instantly, and try to hit them with the blasts that follow. Unfortunately though, it has a very long cooldown in pvp, so use this when you think you can grab them or hit them with the blast, as using this skill means that you will not have access to a grab for the next twenty or so seconds.

Flame Pulverization
Seha drives his gunblade into the ground, causing fire to erupt from the ground around him. Holding this skill causes Seha to continuously blast the ground with fire, and this skill will leave a pool of fire behind that will continually deal damage over time.

This skill has high damage and very good range (with [Phase Focus Blade], the range seems to be about 50% larger, but it is harder to tell as the flame pool doesn't get bigger.), and can be used to stall while you are waiting for your cooldowns. It has damage reduction during the skill, and it also has innate grab resistance. Unfortunately, this skill brings you back to the ground, but there is a technique to eliminate that problem.

In PvP, this skill is an extremely high damage AoE, while also being fast to come out, and you can use it to trade, tank damage, or resist grabs.

EXTRA TECHNIQUE

(This skill has no real English name, so I will refer to it as "Pulv Cancel")
Like [Sky Slash], [Flame Pulverization] can also be cancelled during the skill to get the hitbox out, but it is slightly different in that this is much more useful. You can cancel the aerial version of [Flame Pulverization] immediately in order to get the initial burst of damage in a large AoE around you and slightly below you, and doing this will leave the pool of fire on the floor beneath you. Doing this allows you to stay in the air, and it allows you to basically skip the entire animation for the skill, giving you much more DPS. However, doing this is only good for damage. You will not be able to receive the benefits of the grab resist and damage reduction by doing this, as you will have to cancel the skill immediately. This technique will trade utility for speed.

Finishing Move Inferno Blade
Seha wields his gunblade with extreme force, causing explosions in front of him. After 6 slashes, he smashes the ground in front of him with the most force he can muster, and follows up with an extra one.

This skill is strong, but slow. It's a very strong skill for Seha, being able to deal about 23000% at level 15, but it suffers from not being affected by attack speed. But even with the slow speed, it is definitely worth it to use, because of the damage you get from it.

In PvP, this skill is awkward to use. The aerial version of this skill doesn't exactly combo into itself (as there is some landing lag when Seha lands after the initial blasts), but the ground version is decent. For some reason though, it doesn't do that much damage, so it should really only be used to break someone's super armor to start a combo or force them to use [Force Cancel]

***EDIT***
In the recent Seha balance patch, the expert cube was buffed to include 100% back attack proccing, meaning that you don't have to always be facing their back in order to deal back damage. The speed of the skill was also increased significantly, making it easier to use and much more useful in boss play.

Special Agent

*I'm not going to go over the passive in-depth, but it just involves a stat conversion from Phase Release Damage to Physical Critical Damage, Aerial Critical Damage, and Back Critical Damage.

EX Discharge
Seha focuses his phase power into a single point, triggering explosions that distort space to deal massive damage to enemies in front of him.

This skill is probably the most disappointing EX skill that Seha receives from his Special Agent promotion. This skill loses the homing blast that the normal [Discharge] has, increases the cooldown by a little over 2x, and doesn't really increase the damage at all.

Instead, the skill gains speed, and using this skill will give you a use of [Burst].

I would not recommend this skill for anyone who doesn't have 40% cooldown reduction, and doesn't have very good gear because of the fact that the cooldown increases by so much for so little gain. However, for raid bosses, this skill is better simply because of the fact that it gives a usage of [Burst], allowing you to use it once more to abuse its high damage.

In PvP, it doesn't even give a usage of [Burst], so it really isn't useful at all. It does decent damage, but the cooldown is horrendous, and it's hard to combo from it.

***EDIT***
In the recent balance patch, they basically doubled the damage that this skill does, and decreased the cooldown to a static 7 second cooldown. Also, this skill's cooldown can be reset by using Instant Awakening, making it much more prominent as an EX skill now. Now this skill is definitely worth taking, as the damage and cooldown are very decent now. However, in PvP, it still has the weakness of having a longer cooldown than normal Discharge, giving Seha less Rush stacks, making it much harder to perform and dash around the map.

EX Pulse Cannon
Seha fires precision shots with his gunblade, shooting up to five times. On the ground, Seha will shoot forward, and in the air, Seha will shoot diagonally downward.

This skill is great, especially after the recent patch where this skill was given increased cast speed, so now it is about 50% faster than it used to be. It has very high damage, and with the Expert skill cube, each shot will give you a stacking buff that gives 2% Physical Damage per stack. This buff can also be kept up 100% of the time, which means that on top of the great damage (~7000% at level 5), this skill will basically increase the rest of Seha's attack's damage by 10%, which is amazing. However, you don't want to throw this out whenever you can, because even if it is faster than it is before, it keeps you stationary for a while, leaving you open to attacks and damage. Try to use it when you know you are safe, so that you can use it without the risk of dying.

In PvP, this skill has amazing range and speed. If you use it in the air, it will pop the enemies upwards a little bit (which looks a little strange, but allows you to continue comboing if you use it mid-combo in the air) and it has a deceivingly large hitbox. This skill is great for sniping enemies from very far away, and it is possible to combo off of shots from far away because of his speed.

***EDIT***
This skill was given the Damage Reduction state in a recent patch, allowing Seha to tank hits while in this animation. This means that you can delay the shots in order to take hits for less damage than what you would normally take.

EX Shockwave
Seha triggers a large explosion on the ground in front of him with his phase power.

This skill fixes the one major flaw that [Shockwave] had. It becomes true damage.

Also, it has a massive hitbox, with amazing vertical range and a large circle AoE.

But with the Expert skill cube, the skill gains a damage over time effect with an extra explosion at the end, and it pulls in nearby enemies as well.
This makes the skill MUCH stronger than it used to be (although the pulling-in effect is kind of awkward during team play).

Unfortunately, it has a really long ending animation, so you'll probably just sit there waiting for Seha to finish his anime pose before you can move again (or you could just use the upwards-[Rush] to go back into the air).

In PvP, [EX Shockwave] acts as a restand, as well as having a GIGANTIC hitbox that your opponent has to avoid unless they want to take a lot of damage. The Expert cube effect loses its suction, but the damage is still insane, and the fact that it's a restand makes Seha have one of the easiest times restanding enemies.

Finishing Move Supernova
Seha materializes a supernova with his phase power, and triggers an explosion so powerful that it wipes away the existance of enemies.

This skill looks really funny if you use it in the air, because he just makes a road for himself to walk on.

This skill is actually really slow, taking ~3 seconds for it to actually finish.Or that'd be what I would say, if you couldn't immediately cancel it.
This skill is basically just free damage because of it's technically zero cast time, kind of like the Pulv Cancel or the Spin Cancel, but with a delay, bigger AoE, and more damage.

EXTRA TECHNIQUE

[Finishing Move Supernova] can be canceled as soon as it is used, and after the short delay that would happen if it was used normally, the blast still happens. Once again, this is reminiscent of [Flame Pulverization] where you sacrifice utility (damage reduction) to increase your DPS.